1. Field of the Invention
The invention relates to a vehicle power storage unit constituted of a power storage assembly and a casing containing the power storage unit and coolant for cooling said unit. The invention also relates to and a vehicle incorporating said unit.
2. Description of the Related Art
In recent years, various motor-driven vehicles such as electric vehicles and hybrid vehicles have been widely developed, and there have been increasing demands for secondary batteries providing a high performance, a high reliability, and a high sedulity, as drive power sources or auxiliary power sources for such motor-driven vehicles.
Power sources used as the power source for propelling a motor-driven vehicle or as the auxiliary power source are required to have a high power density. As one of such power sources, there is known a power storage unit constituted of a battery assembly consisting of multiple battery cells connected in series or in parallel and a container containing the battery assembly and coolant for cooling the battery assembly. The container is constituted of a container body having an opening at the upper side and a lid covering said opening. The lid is fixed on the container using fasteners.
Typically, a gas-release valve is provided at each battery cell. This valve is used to discharge the gas produced by electric decomposition of electrolyte solution upon overcharging of the battery cell, or the like, preventing an excessive increase in the internal pressure of the battery cell.
The internal pressure of the container increases due to the gases discharged from the respective battery cells. Therefore, the compressive strength of the container needs to be high. However, if the thickness of the container is increased to increase its compressive strength, its weight increases accordingly, making the power storage unit larger in size and heavier in weight. In view of this, it is necessary to release the gas in the container to the outside.
Japanese Patent Application Publication No. 2005-71674 (JP-A-2005-71674) discloses a battery constituted of a resin case containing electrodes and electrolyte solution. A gas-discharge member is provided at the resin case to release gas, and a gas-discharge pipe for guiding gas to the outside is attached to the gas-discharge outlet of the gas-discharge member.
Based on this structure of JP-A-2005-71674, in the foregoing power storage unit, for example, a gas-discharge pipe may be connected to the lid of the container so that the gas produced by each battery cell is discharged to the outside of the container.
However, if the internal pressure of the container increases sharply due to the gas produced by the respected battery cells, the gas may not be discharged sufficiently through the gas-discharge pipe only. Therefore, the power storage unit needs to be made large in size and heavy in weight.
One option for solving this issue may be to connect two or more gas-discharge pipes to the lid. In this case, however, the manufacturing procedure of the lid becomes complicated and the production cost increases accordingly.